Eating chocolate can boost the level of heart-protecting antioxidants in the blood, but consuming milk at the same time cancels the potential health benefits, according to a new study.

The researchers speculate that milk may also have the same effect on other antioxidant-rich foods, including fruit and green vegetables.

Researchers in Scotland and Italy looked at the body's absorption of an antioxidant found in cocoa, called epicatechin, and a type of flavonoid.

Dark chocolate contains about twice the amount of flavonoids as milk chocolate, so 12 healthy volunteers were given either 100 grams of plain chocolate or 200 grams of milk chocolate. Some were also given 200 ml of milk to drink in the double-blind experiment. The levels of antioxidant in their blood plasma were tested after one, two and four hours.

Maximum benefit

"Those volunteers who had dark chocolate had a 20 per cent increase in antioxidants in their plasma," says Alan Crozier, one of the team at the University of Glasgow. "But those who had milk chocolate, or milk with their dark chocolate, showed no increase in epicatechin plasma levels,"

Four hours after eating the chocolate, all the volunteers' blood antioxidant levels had returned to normal. To gain the maximum potential benefits from chocolate, Crozier suggests it may be advisable to refrain from milk products during that period.

"Presumably the epicatechins are binding to the milk proteins," he told New Scientist . "Dairy products may inhibit the body's absorption of flavonoids from other foods as well."

Antioxidants are involved in lowering the levels of free-radicals in the blood. Prolonged and high-level exposure to free radical has been linked to cardiovascular disease and some cancers.